The use of electrochemical cells with solid electrolytic elements and gas porous platinum electrodes for detecting or measuring the content of oxygen or certain other gaseous compounds in a sample gas is well known. See, for example, U.S. Pat. No. 3,928,161 to McIntyre et al. and U.S. Pat. No. 4,282,078 to Chamberland et al. Materials such as zirconia and yttria-thoria are good oxygen ion conductors at elevated temperatures but are less ionically conductive or essentially ionically non-conductive at temperatures between the elevated temperatures and room temperature. Where the electrodes of such a cell are subjected simultaneously to differing oxygen concentrations, an emf is developed across the cell between the electrodes the value of which can be determined by the Nernst equation as follows: ##EQU1## where emf=sensor output in volts
T=absolute temperature PA1 R=gas constant PA1 F=Farraday constant PA1 P.sub.1 (O.sub.2)=reference gas (oxygen) partial pressure PA1 P.sub.2 (O.sub.2)=sample gas (oxygen) partial pressure PA1 C=cell constant
Similar equations can be developed for other electrochemical systems. The cell constant C is a correction factor which reflects the inability of a particular cell to perform to theoretical limits represented by the preceding term of the equation. Cells used for oxygen concentration determination should be designed so as to consistently exhibit a known, predetermined cell constant value under all operating conditions in order that the bias in emf might be corrected or, preferably, eliminate such bias entirely (and thus the constant C).
A particularly useful application of oxygen concentration measuring apparatus is for the measurement of the oxygen content of exhaust gases from boilers, furnaces, glass furnaces, etc. in order that the combustion or smelting process may be optimally controlled. Closed loop oxygen sensing systems for combustion control in large steam plants used for power generation or industrial heating have been commercially available for at least ten years. Many of the systems available use a zirconia (ZrO.sub.2) based solid electrolyte sensing cell installed in either an in situ or extractive mode. An in situ sensor is physically located within the boiler exhaust gas stream. Extractive systems are located outside the stack and require "plumbing" to carry a representative sample of the flue gas from the stack to the sensor. Currently available commercial units of both designs typically are provided with an auxiliary electric furnace to heat the sensor cell to a satisfactory operating temperature. In in situ sensors, the outer surface of the furnace also typically acts to protect the sensor probe from damage due to the high velocity particles in the flue gas and from dust caking which would affect the accuracy of the sensor
The auxiliary electric furnaces used by currently available oxygen measuring systems have certain undesirable characteristics. They are an added component to the system and represent a significant fraction of its total cost. They are cumbersome to mount around the electrochemical sensor cell and require considerable design work to assure providing proper heating with simultaneous sample gas exchange They tend to be bulky and heavy, particularly when used with an in situ sensor which typically extends a meter or more into a flue or stack gas stream. Moreover, such furnaces consume substantial amounts of electrical power, typically between 200 and 400 watts, to maintain a sensor cell at a conventional 800.degree. C. operating temperature. Lastly they are the least reliable component of the system and are a significant source of repair problems and cost.
Various attempts have been made to eliminate or at least reduce the size and complexity of the auxiliary furnace. For example, U.S. Pat. No. 4,098,650 to Sayles depicts an in situ oxygen measuring device in which the sensing cell is formed with a hollow interior into which is inserted a coil resistance wire heater. The aforesaid U.S. Pat. No. 3,928,161 to McIntyre et al. shows a different configuration for an in situ oxygen detector apparatus incorporating a resistance wire heater located within a tubular member supporting a disk shaped solid electrolyte sensor cell. U.S. Pat. No. 4,334,940 to Habdas et al. depicts yet another solid electrolyte oxygen sensor in which a resistance wire heater is incorporated internally into a tubular support member containing the solid electrolyte sensor cell. While these heater configurations would appear to use less power than auxiliary furnaces, the series resistance windings employed by each would continue to consume significant amounts of electrical power. Moreover, incorporating the heater windings as indicated requires additional manufacturing steps and increasing manufacturing complexity, particularly where the wires are to be threaded through fine holes. Lastly, heating of the solid electrolyte sensor cell would appear to be uneven and unpredictable. The sensor output is dependent upon temperature (see the Nernst equation, above) and failure to maintain the sensor at a specific temperature or to accurately measure the temperature of the sensor will lead to errors in the detector output.